Theory of Antithetic Alternation of Generations. 125 
phyllous stock. But they might equally be regarded as an isolated 
remnant of an ancestral megaphyllous stock with the adaxial 
position of the spore-bearing organs, from which the present 
microphyllous forms have been derived by reduction. The existence 
of relatively large-leaved ancient forms of Sphenophyllales and 
Equisetales supports the latter view. In any case the Ophio- 
glossales do not represent a transition from microphyllous forms 
to the Ferns. 
(4). On the general point of the relation of the “ microphyll ” 
to the “ megaphyll,” there is no evidence of any capacity of the 
microphyll to evolve into the megaphyll. The former seems to be 
most naturally regarded as an organ which by reduction, or by its 
original nature, does not possess the potentiality of further 
evolution. 
The space we have devoted to Professor Bower’s “ Theory of 
the Strobilus ” and the considerations which it suggests must be 
justified by the fact that this theory is the keystone of the author’s 
morphological arch, and upon its validity depends the main thesis 
of the work. We must now return to a consideration of his 
succeeding chapters. 
In dealing with “ Sporangiophores and Sporophylls ” the author 
shows how the use of the non-committing word “ sporangiophore ” 
has become necessary in the light of modern knowledge of the 
variety of spore-bearing organs, all of which certainly cannot be 
reduced to the concept of the sporophyll. Professor Bower considers 
that the sporangiophore is simply a placental out-growth and is 
therefore an organ sni generis not to be homologised with any other 
type of member. In the course of the chapter the author has some 
very wise cautions against the practice, still too common, of reading 
conceptions derived from a study of higher forms into the structural 
features of lower ones. 
The next chapter is concerned with an attempt to show that the 
foliage leaf is a sterilised sporophyll, in opposition to the old notion 
that the flower is a modified foliage shoot, a notion which clearly 
cannot be interpreted directly in terms of descent, because spore¬ 
bearing organs must have pre-existed. But apart from the strobiloid 
theory it is not at all necessary to bind ourselves to one of these 
alternatives. The ancestors of the Pteridophytes must certainly 
have regularly formed sporangia, hut they need not necessarily have 
done so on all their axes, foliar organs or appendages, any more 
than a Thallophyte does at the present day. Given the potentiality of 
forming reproductive organs of a definite kind, it is certain from Klebs’ 
work that the time and position of their formation is primitively 
mainly determined by external stimuli. Later on in the course 
of evolution it becomes practically independent of these stimuli, 
the morphogenic impulse depending almost entirely (as it always 
did partially) upon internal stimuli which in their turn depend upon 
the regular sequence of events in a definite ontogeny. The 
regularity with which the reproductive cells are formed in certain 
positions will therefore naturally vary according to the degree of 
definiteness attained by this sequence, and according to the 
completeness of the shifting of the governing stimuli from external 
to internal factors. Upset the normal ontogenetic sequence, by the 
removal of leaves and the like, and you will upset the normal period 
